Structural, photoluminescence and energy transfer investigations of novel Dy3+ → Sm3+ co-doped NaCaPO4 phosphors for white-light-emitting diode applications

In this study, Dy3+-doped and Dy3+/Sm3+ co-doped NaCaPO4 white-emitting polycrystalline phosphor samples were synthesized using a solid-state reaction method. The samples were characterized by powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Field-emission scanning electron microscopy (FE-SEM), and Photoluminescence (PL) analysis. The phase purity characterization and crystal structural analysis were done using the Rietveld refinement-based FullProf Suite software. The Rietveld refinement result confirms single-phase formation for both Sm3+ and Dy3+/Sm3+ co-doped NaCaPO4 samples with an orthorhombic structure and with a monotonic change in lattice parameters with doping. The PL studies of the Dy3+-doped samples revealed two emission bands. However, at 352 nm, the Dy3+/Sm3+-co-doped samples revealed distinctive emission bands for both ions. The emission peaks at 480 nm (blue) and 573 nm (yellow) are related to the 4F9/2 → 6H15/2 and 4F9/2 → 6H13/2 transitions of Dy3+ ions; however, the emission peaks at 600 nm and 647 nm are attributed to the 4G5/2 → 6H7/2 and 4G5/2 → 6H7/2 transitions of Sm3+ ions. The intensity of the Dy3+ emissions decreased as the Sm3+ levels increased but the emission intensity of the Sm3+ ions increased. The co-doping of Sm3+ ions in Dy3+-doped phosphors results in unique characteristics due to the energy transfer (ET) from Dy3+ → Sm3+ ions. The effectiveness of this ET from Dy3+ → Sm3+ ions is positively correlated with the dopant amounts of the Sm3+ ions. The interaction mechanisms have been identified as dipole-dipole based on Dexter's energy transfer and Readfield's approaches. All decay curves can be adequately fitted via bi-exponential functions, suggesting the movement of energy between Dy3+ → Sm3+ ions. Temperature-dependent PL measurements and CIE color coordinate analysis reveal excellent luminescent properties, making these Dy3+/Sm3+ co-doped phosphors advantageous for white light-emitting diode (WLED) technologies.

Recent progress of effect of crystal structure on luminescence properties of Ce3+–Eu2+ Co-doped phosphors

Currently, the mechanism of Ce³⁺–Eu²⁺ ET is frequently used to obtain color adjustable or white phosphors. Correspondingly, the ET efficiency from Ce³⁺ to Eu²⁺ becomes an important indication of the luminescent properties of phosphors. However, the ET efficiency calculated using the formula does not always match the emission spectra; the transmission efficiency of Ce³⁺ is high, but the emission efficiency of Eu²⁺ is low, depending on our investigation results. In addition to this problem, here we mainly review, on the basis of substantial examples, how to boost the actual ET efficiency of Ce³⁺-to-Eu²⁺ and thus to improve the luminescent properties of phosphors through the rational design of layered crystal structure and the way of selective occupation of activator ions. Moreover, the possible physical mechanisms are proposed.

Effect of crystal structure on luminescence properties of Ce³⁺–Eu²⁺ co-doped phosphors.

Crystal structure, optical spectroscopy and energy transfer properties in NaZnPO4:Ce3+, Tb3+ phosphors for UV-based LEDs

A new series of Ce3+, Tb3+ singly doped and Ce3+/Tb3+ co-doped NaZnPO4 (NZPO) phosphors have been synthesized via a high-temperature solid-state reaction method at 800 °C. The crystal cell structure, luminescence proprieties, energy transfer, and chromaticity coordinates of the as-prepared phosphors were investigated in detail. The photoluminescence spectra of NZPO:Ce3+ phosphors exhibited broad emission in the 300-380 nm range, while under UV excitation, the singly doped NZPO:Tb3+ phosphor showed emission peaks at ∼485-690 nm among which the green emission peak appears at ∼543 nm. The Tb3+ green emission was significantly enhanced almost 20 times via energy transfer from Ce3+ to Tb3+. The energy transfer (ET) mechanism from Ce3+ to Tb3+ in NZPO is identified to be a resonant type via the dipole-dipole interaction mechanism with an ET efficiency of 91%. Intense green emission is obtained at very low Tb3+ concentrations under 285 nm excitation, making NZPO:Ce3+/Tb3+ an efficient UV-excited green phosphor. The NaZnPO4:Ce3+/Tb3+ phosphors are promising UV convertible materials of green light for UV -LEDs applications.

Color tuning in Ca3−xMx(PO4)2:Eu2+ (M = Sr, Ba) phosphors via cation substitution

A series of Ca_3−xM_x(PO₄)₂:Eu²⁺ (M = Sr, Ba) phosphors were prepared via a high-temperature solid-state reaction process.

The influence of Si4+ co-doping on the spectroscopic properties of β-NaCaPO4:Eu2+/Eu3+

A series of luminescent materials based on a sodium calcium phosphate matrix doped with europium and different concentrations of silicon ions was synthesized using the Pechini method.